1. Field of the Invention
The present invention relates to a fixing apparatus mounted on image forming apparatuses such as a copying machine, a printer, and a facsimile, and, more particularly to a fixing apparatus for an image forming apparatus employing an induction heating system.
2. Description of the Background
In recent years, there are fixing apparatuses of an induction heating system used in image forming apparatuses of an electrophotographic system such as a copying machine and a printer. In the fixing apparatuses of the induction heating system, an eddy-current is generated in a metal layer of the fixing apparatus using an induction heating coil to cause the fixing apparatus to generate heat. In the past, there is a device that ON-OFF controls electric power supplied to the induction heating coil to keep the temperature of a fixing apparatus constant.
However, from a viewpoint of quick-start of the image forming apparatus or saving of energy consumption, a fixing apparatus having a small heat capacity may be used. In the case of such a fixing apparatus having a small heat capacity, temperature fluctuation in the fixing apparatus increases when the supply of the electric power to the induction heating coil is simply ON-OFF controlled. Therefore, it is likely that an adverse effect occurs in fixing performance. Thus, there is also a device that adjusts, to more finely control the electric power supplied to the induction heating coil, fluctuation in electric energy supplied to the induction heating coil to decrease a temperature ripple of the fixing apparatus using a CPU.
However, when the electric power supplied to the induction heating coil is adjusted to control the temperature of the fixing apparatus using the CPU, depending on processing speed of the CPU, it is likely that the supplied electric power cannot be instantaneously adjusted and controlled. Because of such a delay in control, it is likely that the temperature ripple of the fixing apparatus expands and, moreover, overshoot is caused. When the temperature of the fixing apparatus is overshot, it is likely that deterioration in an image quality due to high-temperature offset occurs. Furthermore, it is likely that the CPU cannot be controlled. Therefore, it cannot be said that this method is sufficiently safe.
Therefore, as the fixing apparatus of the induction heating system, development of a fixing device for an image forming apparatus is desired that holds a uniform fixing temperature and obtains a fixed image with a high quality even if the fixing apparatus adjusts and controls electric power supplied to an induction heating coil and has a small heat capacity. Moreover, development of a fixing apparatus for an image forming apparatus that is sufficiently safe and has high reliability is desired.